The rapid discovery of human monoclonal antibodies (mAbs) that arise during natural viral infection is central to the Prometheus consortium for developing immunoprophylactic and therapeutic medical countermeasures (MCMs). Our Center of Excellence for Translational Research will focus on developing MCMs for three groups of NIAID Category A priority pathogens (ebolaviruses, Crimean-Congo hemorrhagic fever virus, and hantaviruses). Ebolaviruses, members of the family Filoviridae, cause severe hemorrhagic fever in humans and non-human primates, with human case-fatality rates of up to 90%. As demonstrated by the 2013? 2016 epidemic of Ebola virus disease in West Africa, ebolaviruses pose a significant public health threat. Crimean-Congo hemorrhagic fever virus is a member of the Bunyaviridae family and induces fatal hemorrhaging in up to 50% of human infections. Hantaviruses also belong to the Bunyaviridae family, and infections are associated with over 150,000 cases of disease annually with case-fatality rates >30%. Given the disease severity caused by these viruses, MCMs are urgently needed. The protective efficacy of hyperimmune serum in animal models and humans strongly suggests that human infection and disease by these groups of viruses will be amenable to treatment with mAb-based therapeutics and prophylactics. Project II will facilitate mAb discovery efforts in Project I via the development of molecular tools (Aim 1) required to isolate and characterize candidate therapeutic mAbs from human survivors. Our focus will be on the generation of novel recombinant antigens and BSL-2 viral surrogates to use as probes in single B- cell sorting experiments. These biologics are also vital to the characterization process (Aim 2) that will define mAb epitopes, competition groups, structures, and mechanisms of action, as well as future product assay development in Core B. The data generated by Project II, combined with potency and synergy data from Project I, will allow for a streamlined and mechanistically informed down-selection of the most promising candidate mAbs and mAb combinations for in depth Fc-effector function analyses and optimization (Aim 3) prior to in vivo evaluation of leads by Core C. Project II will also be involved in finalizing the optimal lead immunoprophylactic mAbs to advance in Project III and immunotherapeutic mAbs to enter advanced development within Core B.